3GPP Long Term Evolution, LTE, is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project, 3GPP, to improve the Universal Mobile Telecommunication System, UMTS, standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. In a typical cellular radio system, wireless devices also known as mobile stations and/or user equipment units, UEs, communicate via a radio access network, RAN, to one or more core networks. The Universal Terrestrial Radio Access Network, UTRAN, is the radio access network of a UMTS and Evolved UTRAN, E-UTRAN, is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a wireless device is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a NodeB, NB, in UMTS, and as an evolved NodeB, eNB or eNodeB, in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a wireless device and receiving signals transmitted by a wireless device.
There are numerous solutions addressing increasing demands of capacity and coverage in a mobile wireless network. One technique is a heterogeneous network deployment, wherein low power radio base stations provide enhanced coverage in a macro cell defined by a high power base station. In the following, the term Pico radio base station, Pico RBS, will be used to denominate a low power radio base station. The term macro RBS will be used to denominate a high power radio base station. A heterogeneous network deployment is, for example, found in urban areas where macro RBS are located to provide radio coverage in large areas, e.g. on a roof top, while Pico RBS are situated to provide local coverage near crowded areas, e.g. on building walls or lamp posts. In the heterogeneous deployment situation, there is usually a large number of Pico RBS deployed within the coverage area of one macro RBS.
The Pico RBS is usually configured to provide one or more combinations of radio access technologies over the radio access link, e.g. 3GPP LTE, 3GPP HSPA, 3GPP GSM or IEEE 802.11x, also known as Wi-Fi. Each Pico RBS is connected to the wireless network by means of a backhaul link. In the present disclosure, the backhaul link is a wireless backhaul link set up between the Pico RBS and a backhaul hub in the wireless network. A macro RBS is set up to function as a backhaul hub. The wireless backhaul link can be implemented using microwave radio communication between the Pico RBS and the macro RBS. It is also common to implement the wireless backhaul link using a 3GPP radio access technology, e.g. 3GPP LTE. For such situations, the backhaul link is typically implemented by using a wireless device; also known as user equipment, UE, embedded into the Pico RBS. On the hub side of the backhaul link, the receiving macro RBS handles the connection as a wireless connection to a connected user equipment, UE. Typically more than one Pico RBS will have a backhaul link to the same backhaul hub, e.g. to a macro RBS. Radio resource management functions are used in the backhaul hub to handle scheduling and prioritization of traffic on the backhaul links to Pico RBSs.
There are two basic operating modes in LTE: Frequency Division Duplex, FDD, and Time Division Duplex, TDD. In FDD the downlink and uplink transmissions are separated on different carriers on separate frequencies, while on TDD downlink and uplink transmissions are separated in the time domain. For the scenario with a wireless backhaul link from a Pico RBS to a macro RBS, the same basic operating mode, e.g. TDD, is generally used for the end-user access radio link as well as for the backhaul radio link.
For TDD, the radio resources of a radio frame are configured for uplink or downlink transmission, wherein some sub-frames are allocated for uplink transmission and some sub-frames for downlink transmission. In LTE, a number of configurations, configuration 0-6, have been provided defining the sub-frames allocated for uplink, UL, and downlink, DL, transmission in a radio frame. To reduce interference between downlink and uplink transmissions in different cells, neighboring cells typically use the same downlink/uplink configuration.
When operating in TDD, there is an inherent latency caused by the partitioning of the radio frame in UL and DL sub-frames. On the DL, data for transmission to an end-user wireless device, e.g. user equipment, UE, can arrive during sub-frames configured for uplink transmission. The transmission will then be delayed a couple of Transmission Time Intervals, TTIs, prior to transmission. Correspondingly, for transmission on the uplink, the wireless device needs to request uplink transmission resources and to wait for a DL sub-frame to receive a grant on PDCCH and then also wait for the corresponding UL sub-frame to transmit.
For the scenario where a TDD operating mode is used both for a wireless backhaul link and an end-user access radio link, latencies caused by the radio frame configuration into uplink and downlink sub-frames will be present on both the wireless backhaul link and on the end-user access link. From a data traffic perspective, the latency on the wireless backhaul link is added to the latency on the end-user access link thereby resulting in delayed data delivery and increased need for buffering data.